Increasing an electrical resistance of a resistor by nitridization

ABSTRACT

A method for increasing an electrical resistance of a resistor. A fraction F of an exterior surface of a surface layer of a resistor of a semiconductor structure is exposed to the nitrogen-comprising molecules. An anodization electrical circuit is formed and includes: a DC power supply, an electrolytic solution including nitrogen, and the resistor partially immersed in the electrolytic solution. The DC power supply is activated and generates a voltage output, that causes an electrolytic reaction in the electrolytic solution near the resistor. The electrolytic reaction generates nitrogen ions from the nitrogen in the electrolytic solution. The fraction F is exposed to the nitrogen ions. A portion of the surface layer is nitridized by being reacted with the nitrogen ions at a temperature above ambient room temperature such that an electrical resistance of the resistor is increased.

This application is a divisional application claiming priority to Ser.No. 11/836,308, filed Aug. 9, 2007, which is a divisional of U.S. Pat.No. 7,351,639, issued Apr. 1, 2008, which is a divisional of U.S. Pat.No. 6,730,984, issued May 4, 2004.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention provides a method and structure for increasing anelectrical resistance of a resistor that is located within asemiconductor structure such as a semiconductor wafer, a semiconductorchip, and an integrated circuit.

2. Related Art

A resistor on a wafer may have its electrical resistance trimmed byusing laser ablation to remove a portion of the resistor. For example,the laser ablation may cut slots in the resistor. With existingtechnology, however, trimming a resistor by using laser ablationrequires the resistor to have dimensions on the order of tens ofmicrons. A method and structure is needed to increase the electricalresistance of a resistor on a wafer generally, and to increase theelectrical resistance of a resistor having dimensions at a micron orsub-micron level.

SUMMARY OF THE INVENTION

The present invention provides a method for increasing an electricalresistance of a resistor, comprising the steps of:

providing a semiconductor structure that includes the resistor; and

oxidizing a fraction F of a surface layer of the resistor with oxygenparticles, resulting in the increasing of the electrical resistance ofthe resistor.

The present invention provides an electrical structure, comprising:

a semiconductor structure that includes a resistor; and

oxygen particles in an oxidizing reaction with a fraction F of a surfacelayer of the resistor, wherein the oxidizing reaction increases anelectrical resistance of the resistor.

The present invention provides a method for increasing an electricalresistance of a resistor, comprising the steps of:

providing a semiconductor structure that includes the resistor; and

nitridizing a fraction F of a surface layer of the resistor withnitrogen particles, resulting in the increasing of the electricalresistance of the resistor.

The present invention provides an electrical structure, comprising:

a semiconductor structure that includes a resistor; and

nitrogen particles in an nitridizing reaction with a fraction F of asurface layer of the resistor, wherein the nitridizing reactionincreases an electrical resistance of the resistor.

The present invention provides a method and structure for increasing anelectrical resistance of a resistor on a wafer generally, and forincreasing the electrical resistance of a resistor having dimensions ata micron or sub-micron level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a front cross-sectional view of a semiconductor structurethat includes an electrical resistor, in accordance with embodiments ofthe present invention.

FIG. 2 depicts FIG. 1 at an onset of exposure of a portion of theresistor to oxygen particles.

FIG. 3 depicts FIG. 2 after exposure of the portion of the resistor tothe oxygen particles.

FIG. 4 depicts a front cross-sectional view of a heating chamber thatincludes the semiconductor structure of FIG. 2 and an oxygen-comprisinggas, wherein the heating chamber generates heat that heats thesemiconductor structure, in accordance with embodiments of the presentinvention.

FIG. 5 depicts a front cross-sectional view of a chamber that includesthe semiconductor structure of FIG. 2 and an oxygen-comprising gas,wherein the resistor of the semiconductor structure is heated by adirected beam of radiation or particles, in accordance with embodimentsof the present invention.

FIG. 6 depicts a front cross-sectional view of a plasma chamber thatincludes the semiconductor structure of FIG. 2, in accordance withembodiments of the present invention.

FIG. 7 depicts a front cross-sectional view of an anodization bath inwhich the semiconductor structure of FIG. 2 is partially immersed, inaccordance with embodiments of the present invention.

FIG. 8 depicts a front cross-sectional view of a chemical bath in whichthe resistor of the semiconductor structure of FIG. 2 is immersed, inaccordance with embodiments of the present invention.

FIG. 9 depicts FIG. 2 during exposure of the portion of the resistor tothe oxygen particles, and with the resistor coupled to an electricalresistance measuring apparatus, in accordance with embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a front cross-sectional view of a semiconductorstructure 10 that includes an electrical resistor 14 within asemiconductor substrate 12, in accordance with embodiments of thepresent invention. The electrical resistor 14 includes an electricallyresistive material. The semiconductor structure 10 may include, interalia, a semiconductor wafer, a semiconductor chip, an integratedcircuit, etc. The substrate 12 comprises all portions of thesemiconductor structure 10 (e.g., electronic devices includingsemiconductor devices, wiring levels, etc.) exclusive of the resistor14. The resistor 14 may have any electrical resistance functionalitywithin the semiconductor substrate 12 and accordingly may exist within asemiconductor device, within an electrical circuit, etc. The resistor 14includes an exposed surface 19 having a surface area S.

FIG. 2 illustrates FIG. 1 at an onset of exposure of a portion 15 of theresistor 14 to oxygen particles 20. The oxygen particles 20 may compriseoxygen-comprising molecules (e.g., molecular oxygen O₂, carbon dioxideCO₂, etc.) or oxygen ions, depending on which of several embodiments ofthe present invention is operative, as will be discussed infra. Theoxygen-exposed portion 15 has an oxygen-exposed surface 17 (i.e.; thesurface 17 is exposed to the oxygen particles 20). The resistor 14includes an oxygen-unexposed portion 16 that has an oxygen-unexposedsurface 18 (i.e.; the surface 18 is unexposed to the oxygen particles20). The surface 19 (see FIG. 1) comprises the surfaces 17 and 18 whichhave surface areas S_(E) and S_(U), respectively. Thus the surface areaS of the surface 19 (see FIG. 1) is S_(E)+S_(U). In FIG. 2, theoxygen-unexposed portion 16 and the associated surface 18, if present,gives rise to a “partially exposed” embodiment, since the surface 19will be partially exposed to the oxygen particles 20 (at the surface 17)such that S_(U)>0. The oxygen-unexposed portion 16 and the associatedsurface 18, if not present, gives rise to a “totally exposed”embodiment, since the surface 19 will be totally exposed to the oxygenparticles 20 (at the surface 17) such that S_(U)=0.

FIG. 3 illustrates FIG. 2 after the exposure of the portion 15 of theresistor 14 to the oxygen particles 20. The exposure of the portion 15of the resistor 14 for a finite time of exposure generates an oxidizedregion 22 within the portion 15, wherein an unoxidized portion 24 of theresistor 14 remains. The oxidized region 22 is a fraction F of a surfacelayer of the resistor 14, wherein the surface layer is a region definedas the oxidized region 22 projected to the side surfaces 25 and 26 ofthe resistor 14. The fraction F is in a range of 0<F≦1, wherein 0<F<1corresponds to the “partially exposed” embodiment, and F=1 correspondsto the “totally exposed” embodiment, discussed supra. The oxidizedregion 22 has a thickness t that may increase as the time of exposureincreases or may reach a self-limiting thickness. For oxidationprocesses which are diffusion dominated, the thickness t may vary, interalia, as a square root of the time of exposure. The oxidized region 22increases an electrical resistance of the resistor 14 associated withcurrent flow either in a direction 6 or in a direction 7, in comparisonwith an electrical resistance of the resistor 14 that existed before theoxidized region 22 was formed.

The resistor 14 could be within an integrated circuit and, accordingly,FIG. 3 also shows in of the integrated circuit above the resistor 14.The insulative layer 11 includes an insulative material 13 and anopening 23, wherein the opening 23 which defines the resistor 14 that ispotentially oxidizable in accordance with the present invention. Notethat there may be resistive regions 28 underneath the insulativematerial 13 and thus blocked by the insulative material 13. Accordingly,the underneath or blocked resistive regions 28 are not oxidizable inaccordance with the present invention. Although not explicated ordiscussed in the embodiments described infra, the resistor 14 could bethought of as being “partially exposed” if the total resistor is definedas the resistor 14 in combination with the underneath or blockedresistive regions 28.

The present invention includes five embodiments for oxidizing theresistor 14 to increase the electrical resistance of the resistor 14,namely: thermal oxidation using a heating chamber (FIG. 4); thermaloxidation using a direct beam of radiation or particles (FIG. 5); plasmaoxidation (FIG. 6); anodization (FIG. 7); and chemical oxidation (FIG.8). The following discussion will describe these embodiments and explainhow in situ testing can be used to control the electrical resistanceacquired by the resistor 14 after being exposed to the oxygen particles20 (FIG. 9).

While the five embodiments mentioned supra and discussed infraspecifically describe oxidizing the resistor 14, the five embodimentsmentioned supra and discussed infra are each applicable to changing anthe resistance of the resistor 14 by nitridizing as an alternative tooxidizing. Nitridizing the resistor 14, as opposed to oxidizing theresistor 14, means reacting the resistor 14 with nitrogen particles(instead of with the oxygen particles 20) in a manner that forms anitride of the electrically resistive material of the resistor 14comprises (instead of forming an oxide of electrically resistivematerial that the resistor 14). As with the oxygen particles 20, thenitrogen particles may be in molecular or ionic form depending on theoperative embodiment. “Partially exposed” and “fully exposed”embodiments are applicable to nitridization of the resistor 14, just as“partially exposed” and “fully exposed” embodiments are applicable tooxidation of the resistor 14. Unless noted otherwise herein, allfeatures and aspects of the five embodiments, as discussed infra, applyto nitridization of the resistor 14 just as said all features andaspects of the five embodiments apply to oxidation.

Thermal Oxidation Using a Heating Chamber

FIG. 4 illustrates a front cross-sectional view of a heating chamber 30that includes an oxygen-comprising gas 32 and the semiconductorstructure 10 of FIG. 2, in accordance with embodiments of the presentinvention. The gas 32 includes an oxygen compound such as, inter alia,molecular oxygen (O₂), nitrous oxide (N₂O), carbon dioxide (CO₂), andcarbon monoxide (CO).

The heating chamber 30 is heated to a heating temperature and theresistor 14 is thus oxidized by the gas 32 to form an oxide regionwithin the resistor 14 such as the oxide region 22 depicted supra inFIG. 3. A thickness of the oxidized region (see, e.g., the thickness tof the oxidized region 22 described supra for FIG. 3) increases as atime of exposure of the resistor 14 to the gas 32 increases. FIG. 4exemplifies a “totally exposed” embodiment in which the oxygen-unexposedportion 16 (see FIG. 2) of the resistor 14 does not exist (i.e., S_(U)=0and F=1), and the surface 17 is the total surface 19 (see FIG. 1) thatis oxidized. In FIG. 4, the oxygen concentration in the ambient gas 32and the heating temperature, in combination, should be sufficient tooxidize the resistor 14. Said combinations depend on the chemistry ofthe oxidizing reaction between the resistor 14 and the gas 32. Thus, therequired oxygen concentration and heating temperature depends on amaterial composition of the resistor 14 and the gas 32.

The gas 32 may be non-flowing in the form of a volumetric distributionwithin the heating chamber 30. Alternatively, the gas 32 may be in aflowing form at low flow, wherein the gas 32 contacts the resistor 14.Since the flowing gas 32 originates from a source that is likely to besubstantially cooler than the heating temperature, the oxygen flow rateshould be sufficiently slow as to minimize or substantially eliminateheat transfer from the resistor 14 to the gas 32. Such inhibition ofheat transfer may by any method known to one of ordinary skill in theart. One such method is for the oxygen flow to be slow enough that thedominant mode of said heat transfer is by natural convection rather thanby forced convection. An additional alternative using flowing oxygenincludes preheating the gas 32 to a temperature sufficiently close tothe heating temperature so that said heat transfer is negligible even ifsaid heat transfer occurs by forced convection.

The heating chamber 30 in FIG. 4 includes any volumetric enclosurecapable of heating the semiconductor structure 10 placed therein. Theheat within the heating chamber 30 may be directed toward thesemiconductor structure 10 in the direction 37 from a heat source 34above the semiconductor structure 10. The heat within the heatingchamber 30 may also be directed toward the semiconductor structure 10 inthe direction 38 from a heat source 36 below the semiconductor structure10. Heat directed from the heat source 34 in the direction 37 istransferred to the surface 17 more directly than is heat directed fromthe heat source 36 in the direction 38. Accordingly, the heat directedfrom the heat source 34 in the direction 37 is more efficient forraising the temperature at the surface 17 than is the heat directed fromthe heat source 36 in the direction 38. Either or both of the heatsources 34 and 36 may be utilized in the heating chamber 30. Either orboth of the heat sources 34 and 36 may be a continuous heat source or adistributed array of discrete heat sources such as a distributed arrayof incandescent bulbs. Alternatively, the heating chamber 30 may be afurnace.

Any method of achieving the aforementioned heating temperature in theheating chamber 30 is within the scope of the present invention. Forexample, the semiconductor structure 10 could be inserted into theheating chamber 30 when the heating chamber 30 is at ambient roomtemperature, followed by a rapid ramping up of temperature within theheating chamber 30 until the desired heating temperature is achievedtherein. If the heating temperature is spatially uniform at and near theresistor 14, then the oxidation of the resistor 14 in the direction 37will be spatially uniform such that a thickness of the resultant oxidelayer is about constant (see, e.g., the thickness t of the oxide layer22 in FIG. 3 which is about constant). A spatially nonuniform heatingtemperature which would result in a oxide layer thickness that is notconstant. Both uniform and nonuniform heating temperature distributions,and consequent uniform and nonuniform oxide layer thicknesses, arewithin the scope of the present invention.

Suitable resistor 14 electrically resistive materials for being oxidizedin the heating chamber 30 include, inter alia, one or more ofpolysilicon, amorphous silicon, titanium, tantalum, tungsten, aluminum,silver, copper, or nitrides, silicides, or alloys thereof.

The aforementioned method of oxidizing the resistor 14 using the heatingchamber 30 does not depend on the dimensions of the resistor 14 and isthus applicable if the resistor 14 has dimensions of 1 micron or less,and is likewise applicable if the resistor 14 has dimensions in excessof 1 micron.

As stated supra, thermal nitridization using a heating chamber could beused as an alternative to thermal oxidation using a heating chamber. Ifnitridization is employed, the gas 32 would include, instead of anoxygen compound, a nitrogen compound such as, inter alia, molecularnitrogen (N₂).

Thermal Oxidation Using a Directed Beam of Radiation or Particles

FIG. 5 illustrates a front cross-sectional view of a chamber 40 thatincludes the semiconductor structure 10 of FIG. 2 and anoxygen-comprising gas 42, wherein the resistor 14 of the semiconductorstructure 10 is heated by a directed beam 46 of radiation or particles,in accordance with embodiments of the present invention. The gas 42includes an oxygen compound such as, inter alia, molecular oxygen (O₂),nitrous oxide (N₂O), carbon dioxide (CO₂), and carbon monoxide (CO). Thegas 42 may be non-flowing or flowing as discussed supra in conjunctionwith the gas 32 of FIG. 4

The portion 15 of the resistor 14 is heated to a heating temperature bythe directed beam 46, and the portion 15 is thus oxidized by the gas 32to form an oxide region within the resistor 14 such as the oxide region22 depicted supra in FIG. 3. A thickness of the oxidized region (see,e.g., the thickness t of the oxidized region 22 described supra for FIG.3) increases as a time of exposure of the resistor 14 to the directedbeam 46 increases. The thickness of the oxidized region also increasesas an energy flux of the directed beam 46 increases. The directed beam46 may include radiation (e.g., laser radiation), or alternatively, abeam of particles (e.g., electrons, protons, ions, etc.). The directedbeam 46 must be sufficiently energetic to provide the required heatingof the resistor 14, and a minimum required energy flux of the directedbeam 46 depends on a material composition of the resistor 14.Additionally, the directed beam 46 should be sufficiently focused sothat the aforementioned energy flux requirement is satisfied.

If the directed beam 46 includes laser radiation, then the laserradiation may comprise a continuous laser radiation or a pulsed laserradiation. If the resistor 14 comprises a metal, then the presentinvention will be effective for a wide range of wavelengths of the laserradiation, since a metal is characterized by a continuum of energylevels of the conduction electrons rather than discrete energy levelsfor absorbing the laser radiation.

The directed beam 46, which is generated by a source 44, may be directedto the oxygen-exposed portion 15 of the resistor 14 in a manner that theoxygen-unexposed portion 16 of the resistor 14 exists. For example, thesource 44 may include a laser whose spot size area is less than thesurface area S of the total surface 19 (see FIG. 1) of the resistor 14,and the associated directed beam 46 includes radiation from the laser ofthe source 44. Thus it is possible for the laser beam to traverse lessthan the total surface 19. Similarly, the source 44 may generate thedirected beam 46 as the beam of particles, which impart energy to theresistor 14 and thus heat the resistor 14. The directed beam 46 may belocalized to the surface 17 which requires that the directed beam 46 besufficiently anisotropic; i.e., sufficiently localized to the direction37 by the source 44, which depends on physical and operationalcharacteristics of the source 44. Accordingly, if the directed beam 46is localized to the surface 17, then FIG. 5 would exemplify a “partiallyexposed” embodiment in which the oxygen-unexposed portion 16 (see FIG.2) exists (i.e., S_(U)>0 and F<1). Alternatively, FIG. 5 may alsoexemplify a “totally exposed” embodiment in which the oxygen-unexposedportion 16 (see FIG. 2) does not exist (i.e., S_(U)=0 and F=1), sincethe directed beam 46 could be directed to the total surface 19. Thus,FIG. 4 exemplifies either a “totally exposed” (F=1) or a “partiallyexposed” (F<1) embodiment in which the oxygen-unexposed portion 16 (seeFIG. 2) may or may not exist. A spatial extent of partial or totalexposure to, and associated reaction with, the oxygen-comprising gas 42may be controlled by adjusting the size (i.e., area) of the directedbeam 46 and/or by scanning the directed beam 46 across portions of thetotal surface 19 (see FIG. 1).

In FIG. 5, the oxygen concentration in the gas 32 and the heatingtemperature, in combination, should be sufficient to oxidize theresistor 14, and depends on the chemistry of the oxidizing reactionbetween the resistor 14 and the gas 32 as discussed supra in conjunctionwith FIG. 4. An ability to achieve the required temperature depends onthe directed beam 46 being sufficiently energetic so as to impart enoughenergy to the portion 15 of the resistor 14 to facilitate the heatingand consequent oxidation of the portion 15. The energy of the directedbeam 46 is controlled at its source 44.

As stated supra, an advantage of using the directed beam 46 of FIG. 5instead of the heating chamber 30 of FIG. 4 to heat the resistor 14 isthe ability to heat less than the total exposed surface area 19 of theresistor 14. Another advantage is that said heating of the semiconductorstructure 10 by the heating chamber 30 could potentially damagethermally-sensitive portions of the semiconductor structure 10 whichcannot tolerate the temperature elevation caused by the heating chamber30. In contrast, the localized heating by the directed beam 46advantageously does not expose said thermally-sensitive portions of thesemiconductor structure 10 to potential thermally-induced damage.

Suitable resistor 14 electrically resistive materials for being oxidizedwhile being heated by the directed beam 46 include, inter alia, one ormore of polysilicon, amorphous silicon, titanium, tantalum, tungsten,aluminum, silver, copper, or nitrides, silicides, or alloys thereof.

If the directed beam 46 is required to be confined to the surface 19(see FIG. 1) of the resistor 14 (i.e., if the directed beam 46 shouldnot strike any surface of the resistor 14 other than the surface 19),then dimensions of the surface 19 should be no smaller than a smallestsurface area on which the directed beam 46 could be focused. Forexample, if the directed beam 46 includes laser radiation and the source44 includes a laser, then the dimensions of the portion 15 of theresistor 14 may be no smaller than a laser spot dimension. Since withcurrent and future projected technology, laser spot dimensions of theorder of 1 micron or less are possible, the portion 15 of the resistor14 may have dimensions of 1 micron or less (to an extent possible withprevailing laser technology at a time when the present invention ispracticed), as well as dimensions exceeding 1 micron, when the directedbeam 46 includes the laser radiation.

As stated supra, thermal nitridization using a directed beam ofradiation or particles could be used as an alternative to thermaloxidation using a directed beam of radiation or particles. Ifnitridization is employed, the gas 42 would include, instead of anoxygen compound, a nitrogen compound such as, inter alia, molecularnitrogen (N₂).

Plasma Oxidation

FIG. 6 illustrates a front cross-sectional view of a plasma chamber 50that comprises the semiconductor structure 10 of FIG. 2, in accordancewith embodiments of the present invention. The plasma chamber 50includes an electrode 54 and an electrode 55. The semiconductorstructure 10 has been disposed between the electrode 54 and theelectrode 55. The plasma chamber 50 also includes oxygen ions 52 whichare formed in generation of a plasma gas, as will be explained infra.

A neutral gas within the plasma chamber 50 includes an oxygen compoundsuch as, inter alia, molecular oxygen (O₂), nitrous oxide (N₂O), carbondioxide (CO₂), and carbon monoxide (CO). Inasmuch as a plasma gas willbe formed from the neutral gas, the plasma chamber 50 may also includeone or more noble gases (e.g., argon, helium, nitrogen, etc.) to performsuch functions as: acting as a carrier gas, providing electric chargeneeded for forming ionic species of the plasma, assisting in confiningthe plasma to within fixed boundaries, assisting in developing a targetplasma density or a target plasma density range, and promoting excitedstate plasma lifetimes.

A power supply 56 generates an electrical potential between theelectrode 54 and the electrode 55. The power supply 56 may be of anytype known to one skilled in the art such as, inter alia: a radiofrequency (RF) power supply; a constant voltage pulsed power supply(see, e.g., U.S. Pat. No. 5,917,286, June 1999, Scholl et al.); and adirect current (DC) voltage source (see, e.g., U.S. Pat. No. 4,292,384,September 1981, Straughan et al.). Pertinent characteristics of thepower supply 56 are in accordance with such characteristics as are knownin the art. For example, a RF power supply may include, inter alia, aradio frequency selected from a wide range of frequencies such as acommonly used frequency of 13.56 Hz. The power requirements of the RFpower supply depends on the surface area 17 of the resistor 14 and isthus case dependent. For example, a typical range of power of the RFpower supply may be, inter alia, between about 100 watts and about 2000watts.

The electrical potential generated by the power supply 56 ionizes theneutral gas to form a plasma between the electrode 54 and the electrode55, wherein the plasma comprises electrons and ions, and wherein aplasma ion polarity depends on the particular neutral gas within theplasma chamber 50. For example, if the neutral gas includes molecularoxygen, then a three-component plasma may be formed including electrons,positive oxygen ions, and negative oxygen ions, such that in the glowdischarge a predominant positive ion is O₂ ⁺ and a lesser positive ionicspecies is O⁺. See U.S. Pat. No. 5,005,101 (Gallagher et al.; April1991; col. 6, lines 1-12).

In FIG. 6, a DC power supply 57 has terminals 58 and 59, wherein theterminal 58 is positive with respect to a ground 51, and the terminal 59is negative with respect to the terminal 58. The DC power supply 57generates an electric field that is directed from the electrode 54 tothe electrode 55, and the electric field is capable of acceleratingpositive ions from the electrode 54 toward the electrode 55 in thedirection 37. Accordingly, if the oxygen ions 52 are positive oxygenions (e.g., O₂ ⁺), then the electric field accelerates the oxygen ions52 of the plasma toward the electrode 55 causing the oxygen ions 52 tostrike the portion 15 of the resistor. If the oxygen ions 52 aresufficiently energetic (i.e., if the oxygen ions 52 have a minimum orthreshold energy) as required to oxidize the portion 15 of the resistor14, then the oxygen ions 52 will so oxidize the portion 15 and thus forman oxidized region within the resistor 14, such as the oxidized region22 depicted supra in FIG. 3. A thickness of the oxidized region (see,e.g., the thickness t of the oxidized region 22 described supra for FIG.3) increases as a time of exposure of the resistor 14 to the acceleratedoxygen ionic species 52 increases.

If the oxygen ions 52 are negative oxygen ions to be accelerated towardthe resistor 14 and reacted with the resistor 14, then the polarities ofthe terminals 58 and 59 should be reversed (i.e., the terminals 58 and59 should have negative and positive polarities, respectively). A factorin determining whether positive or negative oxygen ions 52 are to bereacted with the resistor 14 includes consideration of the chemicalreactions between said accelerated oxygen ions 52 and the electricallyresistive material of the resistor 14, since characteristics of saidchemical reactions (e.g., reaction energetics, reaction rate, etc.) maybe a function of the polarity of the reacting ionic oxygen species 52.Nonetheless, if negative oxygen ions 52 of the plasma are accelerated bythe DC power supply 57 toward the resistor 14, then electrons of theplasma will also be accelerated toward the resistor 14, which in somesituations may result in undesirable interactions between said electronsand the resistor 14. Thus, each of the aforementioned considerations(e.g., material of the resistor 14, characteristics of the chemicalreactions between the oxygen ions 52 and the resistor 14, etc.) must beconsidered when choosing the neutral gas and choosing which ionicspecies 52 to react with the resistor 14.

The accelerated oxygen ions 52 transfer energy to the resistor 14 toprovide at least the threshold energy required for effectuating thechemical reaction between the oxygen ions 52 and the resistor 14, andsuch energy transferred substitutes for thermal energy (i.e., heat)provided by the heating chamber 30 of FIG. 4, or by the directed beam 46of radiation or particles of FIG. 5, to the resistor 14. A voltageoutput of the DC power supply 57 must be sufficient to accelerate theoxygen ions 52 to at least the aforementioned threshold energy.

FIG. 6 exemplifies a “totally exposed” embodiment in which theoxygen-unexposed portion 16 (see FIG. 2) of the resistor 14 does notexist (i.e., S_(U)=0 and F=1), and the surface 17 is the total surface19 (see FIG. 1) that is oxidized in the plasma chamber 50.

While FIG. 6 depicts a particular plasma chamber 50 configuration foroxidizing the resistor 14, any plasma configuration known to one ofordinary skill in the art may be used.

Suitable resistor 14 electrically resistive materials for being subjectto plasma oxidation include, inter alia, one or more of polysilicon,amorphous silicon, titanium, tantalum, tungsten, aluminum, silver,copper, or nitrides, silicides, or alloys thereof.

The aforementioned method of oxidizing the resistor 14 using plasmaoxidation does not depend on the dimensions of the resistor 14 and isthus applicable if the resistor 14 has dimensions of 1 micron or less,and is likewise applicable if the resistor 14 has dimensions in excessof 1 micron.

As stated supra, plasma nitridization using a directed beam of radiationor particles could be used as an alternative to plasma oxidation using adirected beam of radiation or particles. If nitridization is employed,the neutral gas within the plasma chamber 50 would include, instead ofan oxygen compound, a nitrogen compound such as, inter alia, molecularnitrogen (N₂).

Anodization

FIG. 7 illustrates a front cross-sectional view of an anodization bath60, in accordance with embodiments of the present invention. Generally,anodizing a first conductive material such as a semiconductor or metalrequires immersing into an electrolytic solution both the firstconductive material and a second conductive material, and passing a DCcurrent at a sufficient voltage through the electrolytic solution.

An anodization electrical circuit 69 includes a DC power supply 64, anelectrolytic solution 61 which includes oxygen, the semiconductorstructure 10 of FIG. 2 wherein the resistor 14 is partially immersed inthe electrolytic solution 61, and an electrode 63 partially immersed inthe electrolytic solution 61. “Partially immersed” includes “totallyimmersed” (i.e., 100% immersed) as a special case. The resistor 14 ismade of the electrically resistive material which includes the firstconductive material that serves as an anode, and the electrode 63 ismade of the second conductive material that serves as a cathode. Thesecond conductive material of the cathode may include any inert metal(e.g., platinum) that does not react with the electrolytic solution 61.The resistor 14 is made anodic by electrically coupling the resistor 14to a positive terminal 65 of the DC power supply 64. The electrode 63 ismade cathodic by electrically coupling the electrode 63 to a negativeterminal 66 of the DC power supply 64. The anodization may be performedat or above ambient room temperature. A thickness of an oxide filmformed with the resistor 14 is a function of a voltage output from theDC power supply 64 and the current density in the anodization circuit69. The specific voltage and current density is application dependentand would be selected from known art by one of ordinary skill in theart. For example, an anodization of tantalum or tantalum nitride atambient room temperature and at with a current density of about 0.1milliamp/cm² in an electrolytic solution of citric acid will generate anoxide (i.e., tantalum pentoxide Ta₂O₅) film thickness of 20 Å per volt.Thus for an applied voltage of about 25 volts, the Ta₂O₅ film thicknessis about 500 Å.

Suitable resistor 14 electrically resistive materials for being anodizedinclude, inter alia, Suitable cathode 63 materials include, inter aliatantalum, titanium, polysilicon, aluminum, tungsten, nitrides thereof,and alloys thereof. A electrolyte containing oxygen that can be useddepends on the electrically resistive material to be anodized and istherefore case specific. Thus, any electrolyte containing oxygen that iscompatible with said electrically resistive material may be selected aswould be known or apparent to one of ordinary skill in the art.

Upon activation of the DC power supply 64 (i.e., the DC power supply 64is turned on), and under the voltage output (and the associated current)from the DC power supply 64, an electrolytic reaction occurs at thesurface 17 of the resistor 14 to generate hydrogen ions, electrons, andoxygen ions 62 from the electrolytic solution. The oxygen ions 62chemically react with the portion 15 of the resistor 14 such that anoxidized region, such as the oxidized region 22 depicted supra in FIG.3, forms within the portion 15 of the resistor 14. The generatedhydrogen ions and electrons combine at the cathode 63 to form hydrogengas.

FIG. 7 shows the portion 16 of the resistor 14 above an electrolytelevel 67. Accordingly, FIG. 7 may exemplify a “partially exposed”embodiment in which the oxygen-unexposed portion 16 (see FIG. 2) exists(i.e., S_(U)>0 and F<1). Alternatively, FIG. 7 may also exemplify a“totally exposed” embodiment in which the oxygen-unexposed portion 16(see FIG. 2) does not exist (i.e., S_(U)=0 and F=1) if the resistor 14is totally immersed in the electrolytic solution 61. Thus, FIG. 7exemplifies either a “partially exposed” embodiment or a “totallyexposed” embodiment in which the oxygen-unexposed portion 16 (see FIG.2) exists or does not exist, respectively.

A thickness of the oxidized region (see, e.g., the thickness t of theoxidized region 22 described supra for FIG. 3) increases as a time ofthe electrolytic reaction increases. As the thickness of the oxidizedregion increases, a current drawn by the anodizing bath 60 decreases dueto increasing isolation of the portion 15 of the resistor 14 from theelectrolytic solution 61 as the thickness of the oxidized layerincreases. For certain resistor 14 materials (e.g., aluminum), theanodization process may eventually self terminate, because said currentis eventually reduced to a negligible value.

The aforementioned method of oxidizing the resistor 14 using anodizationdoes not depend on the dimensions of the resistor 14 and is thusapplicable if the portion 15 of the resistor 14 has dimensions of 1micron or less, and is likewise applicable if the portion 15 of theresistor 14 has dimensions in excess of 1 micron.

As stated supra, anodization that causes nitridization of the resistor14 could be used as an alternative to anodization that causes oxidationof the resistor 14. If anodization with nitridization is employedinstead of anodization with oxidation, then the electrolytic solution 61would include nitrogen instead of oxygen. An electrolyte containingnitrogen that can be used depends on the electrically resistive materialto be anodized and is therefore case specific. Thus, any electrolytecontaining nitrogen that is compatible with said electrically resistivematerial may be selected as would be known or apparent to one ofordinary skill in the art.

Chemical Oxidation

FIG. 8 illustrates a front cross-sectional view of a chemical bath 70,in accordance with embodiments of the present invention. The chemicalbath 70 comprises a chemical solution 71. The semiconductor structure 10of FIG. 2 is immersed in the chemical solution 71. The chemical solution71 includes oxygen particles 72 in such form as oxygen-comprising liquidmolecules, oxygen ions, or an oxygen-comprising gas (e.g., oxygen gas orozone gas) dissolved in the chemical solution 71 under pressurization.The oxygen particles 72 chemically react with the resistor 14 to form anoxidized region within the resistor 14 such as the oxidized region 22depicted supra in FIG. 3. A thickness of the oxidized region (see, e.g.,the thickness t of the oxidized region 22 described supra for FIG. 3)increases as a time of the chemical reaction increases. The chemicalreaction may be exothermic or endothermic, depending on the electricallyresistive material of the resistor 14 and the oxygen particles 72. Ifthe chemical reaction is endothermic, an addition of a sufficient amountof heat is required. Additionally, a suitable catalyst may be utilizedto accelerate the chemical reaction. The catalyst may be any catalystknown to one of ordinary skill in the art for the particular chemicalreaction.

Suitable resistor 14 electrically resistive materials for beingchemically oxidized include, inter alia, copper, tungsten, aluminum,titanium, nitrides thereof, and alloys thereof. Suitable chemicalsolutions 71 include, inter alia, hydrogen peroxide, ferric nitrate,ammonium persulphate, etc.

FIG. 8 shows the resistor 14 as totally immersed in the chemicalsolution 71, which exemplifies a “totally exposed” embodiment in whichthe oxygen-unexposed portion 16 (see FIG. 2) of the resistor 14 does notexist (i.e., S_(U)=0 and F=1), and the surface 17 is the total surface19 (see FIG. 1) that is oxidized in the chemical solution 71.Nonetheless, the resistor 14 could be rotated 90 degrees (within thecross-section plane illustrated in FIG. 8) and moved upward in adirection 75 such that a portion of the resistor 14 would be above thelevel 77 of the chemical solution 71 just as the portion 16 is above theelectrolyte level 67 in FIG. 7. Under such 90 degree rotation and upwardmovement, FIG. 8 would represent a “partially exposed” embodiment inwhich the oxygen-unexposed portion 16 (See FIG. 2) exists (i.e., S_(U)>0and F<1). Accordingly, FIG. 8 exemplifies either a “partially exposed”embodiment or a “totally exposed” embodiment in which theoxygen-unexposed portion 16 (see FIG. 2) exists or does not exist,respectively.

The aforementioned method of oxidizing the resistor 14 using chemicaloxidation does not depend on the dimensions of the resistor 14 and isthus applicable if the resistor 14 has dimensions of 1 micron or less,and is likewise applicable if the resistor 14 has dimensions in excessof 1 micron.

As stated supra, chemical nitridization of the resistor 14 could be usedas an alternative to chemical oxidation of the resistor 14. If chemicalnitridization is employed instead of chemical oxidation, then thechemical solution 71 would include nitrogen particles instead of theoxygen particles 72.

Resistance Testing

The resistor 14 may be tested prior to being oxidized or nitridized,while being oxidized or nitridized (i.e., in situ), and/or after beingoxidized or nitridized. The resistance testing may be accomplished by aconventional test apparatus, such as with a four-point resistance testhaving four contacts to the resistor with two of the contacts coupled toa known current source outputting a current I and the other two contactscoupled to a voltage meter that measures a voltage V across theresistance to be determined, and the measured resistance is thus V/I.Alternatively, the resistance testing may be accomplished with an inlinemeasuring circuit within the same integrated circuit that includes theresistor, wherein the measuring circuit is coupled to instrumentationthat outputs the measured resistance.

FIG. 9 illustrates FIG. 2 during exposure of the portion 15 of theresistor 14 to the oxygen particles 20, and with the resistor 14 coupledto an electrical resistance measuring apparatus 85. The electricalresistance measuring apparatus 85 may include the conventional testapparatus or the inline measuring circuit, mentioned supra. Theelectrical resistance measuring apparatus 85 may be conductively coupledto surfaces 81 and 82 of the resistor 14 by conductive interconnects(e.g., conductive wiring) 86 and 87, respectively. Accordingly, theelectrical resistance measuring apparatus 85 is capable of measuring anelectrical resistance of the resistor 14 (before, during, and afteroxidation or nitridization of the resistor 14) associated with currentflowing in the direction 7 through the resistor 14. Alternatively, theelectrical resistance measuring apparatus 85 may be used to measure anelectrical resistance of the resistor 14 associated with current flowingin the direction 6 through the resistor 14 (before, during, and afteroxidation or nitridization of the resistor 14) if the conductiveinterconnects 86 and 87 are coupled to bounding surfaces 83 and 84 ofthe resistor 14 instead of to the surfaces 81 and 82, respectively. Thesurface 83 in FIG. 9 corresponds to the surface 19 in FIG. 1. In FIG. 9,the resistor 14 includes an oxidized (or nitridized) region 21, whichcorresponds to the oxidized (or nitridized) region 22 of FIG. 3. Thesemiconductor structure 10 is within an oxidizing (or nitridizing)environment 80, which includes any oxidizing (or nitridizing)environment within the scope of the present invention such, inter alia,the heating chamber 30 of FIG. 4, the chamber 40 of FIG. 5, the plasmachamber 50 of FIG. 6, the anodization bath 60 of FIG. 7, and thechemical bath 70 of FIG. 8. The electrical resistance measuringapparatus 85 is any apparatus, as is known to one of ordinary skill inthe art, capable of measuring an electrical resistance of the resistor14.

The following discussion describes how the electrical resistancemeasuring apparatus 85 of FIG. 9 can be used for in situ testing tocontrol the electrical resistance acquired by the resistor 14 afterbeing exposed to the oxygen particles 20. The following discussionapplies to any of the embodiments described supra (i.e., thermaloxidation or nitridization using a heating chamber, thermal oxidation ornitridization using a directed beam of radiation or particles, plasmaoxidation/nitridization, anodization, and chemicaloxidation/nitridization).

Let R₁ denote an electrical resistance of the resistor 14 prior to beingoxidized or nitridized. Let R₂ denote a final electrical resistance ofthe resistor 14 (i.e., an electrical resistance of the resistor 14 afterbeing oxidized or nitridized). Let R_(t) denote a predetermined targetelectrical resistance with an associated resistance tolerance ΔR_(t) forthe resistor 14 after the oxidation (or nitridization) has beencompleted (i.e., it is intended that R₂=R_(t) within the toleranceΔR_(t)). The target electrical resistance R_(t) is applicationdependent. For example, in an analog circuit R_(t) may be a function ofa capacitance in the circuit, wherein for the given capacitance, R_(t)has a value that constrains the width of a resonance peak to apredetermined upper limit. In practice, the predetermined resistanceR_(t), together with the associated resistance tolerance ΔR_(t), may beprovided for the intended application.

The resistor 14 may have its electrical resistance tested during orafter the exposure of the resistor 14 to the oxygen particles 20. Asstated supra, the thickness t of the oxidized (or nitridized) region 22(see FIG. 3) increases as the time of said exposure increases, and theelectrical resistance of the resistor 14 increases as the thickness tincreases. Thus, the final electrical resistance may be controlled byselection of the time of exposure. The time of exposure may be selectedbased on any method or criteria designed to obtain R₂ as being withinR_(t)±Δ_(t) (i.e., R_(t)−Δ_(t)≦R₂≦R_(t)+ΔR_(t)). For example,calibration curves derived from prior experience may be used fordetermining the time of exposure that results in R₂ being withinR_(t)±ΔR_(t).

An iterative testing procedure may be utilized such that the electricalresistance of the resistor 14 is tested during the exposing of theresistor 14 to the oxygen particles 20 and thus during the oxidizing (ornitridizing) of the resistor 14. The testing during the exposing of theresistor 14 to the oxygen particles 20 determines continuously orperiodically whether R₂″ is within R_(t)±ΔR_(t), wherein R₂″ is thelatest resistance of the resistor 14 as determined by the testing. Thetesting is terminated if R₂″ is within R_(t)±ΔR_(t) or if(R₂″−R₁)(R_(t)−R₂″)≦0.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. Accordingly,the appended claims are intended to encompass all such modifications andchanges as fall within the true spirit and scope of this invention.

1. A method for increasing an electrical resistance of a resistor,comprising the steps of: providing a semiconductor structure thatincludes the resistor; forming an anodization electrical circuit whichincludes: a DC power supply, an electrolytic solution comprisingnitrogen, the resistor partially or totally immersed in the electrolyticsolution such that a fraction F of an exterior surface of a surfacelayer of the resistor is immersed in the electrolytic solution, and acathode partially immersed in the electrolytic solution, wherein theresistor is electrically coupled to a positive terminal of the DC powersupply such that the resistor serves as an anode, and wherein thecathode is electrically coupled to a negative terminal of the DC powersupply; activating the DC power supply such that the DC power supplygenerates a voltage output, wherein the voltage output causes anelectrolytic reaction in the electrolytic solution near the resistor,and wherein the electrolytic reaction generates nitrogen ions from thenitrogen in the electrolytic solution; exposing the fraction F of theexterior surface of the surface layer of the resistor to the nitrogenions; and nitridizing a portion of the surface layer by reacting saidportion with the nitrogen ions at a temperature above ambient roomtemperature such that an electrical resistance of the resistor isincreased, wherein an exterior surface of said portion consistsessentially of the fraction F of the exterior surface of the surfacelayer.
 2. The method of claim 1, wherein a dimension of the resistordoes not exceed about 1 micron.
 3. The method of claim 1, wherein F<1.4. The method of claim 1, wherein F=1.
 5. The method of claim 1, whereinthe resistor includes an electrically resistive material selected fromthe group consisting of polysilicon, amorphous silicon, titanium,tantalum, tungsten, aluminum, silver, copper, nitrides thereof,silicides thereof, and alloys thereof.
 6. The method of claim 1, whereinthe resistor partially immersed in the electrolytic solution.
 7. Themethod of claim 1, wherein the resistor totally immersed in theelectrolytic solution.
 8. The method of claim 1, wherein the methodfurther comprises: predetermining a target resistance R_(t) and anassociated tolerance ΔR_(t) for the electrical resistance of theresistor; and testing the resistor during the nitridizing step todetermine whether the electrical resistance of the resistor is withinR_(t)+ΔR_(t).
 9. The method of claim 8, wherein if during the testingstep the electrical resistance of the resistor is determined to not bewithin R_(t)±ΔR_(t) then the method further comprises: iterating suchthat each iteration of the iterating includes additionally executing theexposing and nitridizing steps and additionally testing the resistorduring the nitridizing step to determine whether R₂″ is withinR_(t)±ΔR_(t), wherein R₂″ is a latest value of the electrical resistanceof the resistor as determined by said testing; and ending the iteratingif R₂″ is within R_(t)±ΔR_(t) or if (R₂″−R₁)(R_(t)−R₂″)<0, wherein R₁ isa latest value of the determined electrical resistance of the resistorimmediately prior to said testing.
 10. The method of claim 8, whereinsaid ending the iterating comprises satisfying R₂″ being withinR_(t)±ΔR_(t).
 11. The method of claim 10, wherein the method furthercomprises determining from a calibration curve the time of exposure thatresults in the electrical resistance of the resistor being withinR_(t)±ΔR_(t) as a result of said nitridizing, and wherein saidnitridizing is performed for the determined time of exposure.
 12. Themethod of claim 8, wherein said ending the iterating comprisessatisfying (R₂″−R_(t))(R_(t)−R₂″)<0.
 13. The method of claim 8, whereinsaid testing comprises continuously testing the resistor during thenitridizing step.